This is one area where Amarr win as far as I'm concerned. I had serious doubts when I was training for them.. That would be before the expanders change where the Impel would actually have a negative number for it's 'top speed'...also before rigs.
Sure am glad I trained them now tho. :)
Impel fits 12 Giant Secure Containers with t2 expanders and 2 cargo rigs, this on top of having t2 resists on cruiser level hitpoints PLUS being able to fit a MWD....it awesome. :P It could tank very nice too if you setup for it, no doubt...
The Prorator is more of a jack of all trades. Maybe not the fastest, or having the biggest cargo bay, but you can set it up to do anything pretty much, and it will do it well.
Last thing, it's just a guess, but I think the Providence with a skilled CS /w armor ganglinks would be the "toughest" freighter since it has the most armor HPs. Could be wrong here tho..and probably am since sheilds are overpowered.